In recent years, because of problems such as global warming and depletion of petroleum resources, and from the viewpoint of carbon neutrality, use of biomass as an alternative to petroleum products has been attracting attention. In particular, production of ethanol and chemical products from non-edible cellulose-containing biomass, which does not compete with food, has been expected.
For production of ethanol and chemical products from cellulose-containing biomass, monosaccharides produced by hydrolysis of cellulose and hemicellulose, which are polysaccharides, in the biomass are used to perform fermentation. Sugar liquids derived from cellulose-containing biomass contain, in addition to sugars, fermentation-inhibiting substances. Decreases in the yields of fermentation products due to such fermentation-inhibiting substances have been problematic.
As a method of removing the fermentation-inhibiting substances while concentrating the sugars to obtain a purified sugar liquid suitable for fermentation, a method in which a nanofiltration membrane and/or reverse osmosis membrane is/are used, wherein the fermentation-inhibiting substances are removed from the permeate side of the membrane(s), is known (WO 2010/067785).
As a method of washing an ultrafiltration membrane, nanofiltration membrane, or reverse osmosis membrane to remove its fouling, thereby efficiently removing fermentation-inhibiting substances, a method in which warm water with a high pH is used for effective washing is known (WO 2012/111794).
However, in the method of WO 2010/067785, long-term operation causes fouling of the membrane(s), which is problematic.
WO 2012/111794 describes alkali washing in which alkaline warm water is used to increase the effect to remove organic substances. However, this method was found to be problematic since, even when sufficient washing can be achieved by alkali washing of an ultrafiltration membrane after filtration treatment of a saccharified liquid derived from cellulose-containing biomass, a similar effect cannot necessarily be obtained for a nanofiltration membrane or a reverse osmosis membrane, and repeated use of such a membrane causes a decrease in the washing effect.
This is thought to be due to the fact that nanofiltration membranes and reverse osmosis membranes, unlike ultrafiltration membranes, cause concentration of inorganic ions derived from cellulose-containing biomass in the primary side, and that the concentrated inorganic ions act as fouling substances. It is generally known that, after processing of a solution containing a large amount of organic substances using a membrane, the membrane may be subjected to alkali washing and then acid washing. However, no washing effect could be obtained for nanofiltration membranes and reverse osmosis membranes after processing of a saccharified liquid derived from cellulose-containing biomass.
It could therefore be helpful to provide a method of obtaining a purified sugar liquid by removal of fermentation-inhibiting substances from a saccharified liquid derived from cellulose-containing biomass, using a nanofiltration membrane and/or reverse osmosis membrane, wherein fouling of the membrane(s) generated by the influence of organic substances and inorganic ions is removed by washing, thereby enabling repeated use of the membrane(s) and efficient removal of the fermentation-inhibiting substances.